As a technique for detecting a defect on and in an object, such as a concrete or steel structure, a laser ultrasonic method is commonly known. In this method, while an elastic wave is induced in an object to be examined, laser light is cast onto the object and the thereby reflected light is detected with a laser interferometer to measure the displacement of the surface of the object. If a defect is present, the displacement due to the elastic wave discontinuously changes at the location of the defect. Accordingly, the defect can be detected by measuring the distribution of the displacement. However, since the detection laser (probe laser) of the laser interferometer is a point-like beam, it is necessary to scan the entire examination area of the object to be examined. Such an operation requires a considerable amount of time.
As an improved version of this technique, a method which employs electronic speckle pattern interferometry has been proposed. In this method, while an elastic wave is induced in an object to be examined, the entire examination area of the object is illuminated with laser light to generate an interference pattern, and the surface displacement is collectively measured over the entire examination area based on the interference pattern. In Patent Literature 1, this method is applied to an examination of an object having a rough surface. The entire examination area of an object to be examined is illuminated with laser light produced by expanding a laser beam using an expander. The laser light is scattered by the rough surface, forming a light-and-dark pattern called the “speckle” due to the mutual interference of the scattered rays of light. This speckle pattern is made to interfere with reference laser light branched from the illuminating laser beam, and an interference image is taken with a CCD camera or similar device. Two such images are respectively taken before and after the occurrence of a displacement on the surface of the object due to the externally given elastic wave. The distribution of the displacement on the examination area is calculated from those two images. By such a technique, the measurement of the entire examination area can be performed at one time, and a defect on or in the object can be detected based on the measured displacements.